Transfer of heat from superheated vapor in a condensing heat exchanger



1962 H. Q. nusum 3,063,681

TRANSFER OF HEAT FROM SUPERHEATED VAPOR IN A CONDENSING HEAT EXCHANGER Filed July 23, 1956 2 Sheets-Sheet 1 INVENTOR:

Z: 47 HOWARD o. DUGUID 1 1:.

HIS ATTORNEY Nov. 13, 1962 H Q. DUGUID 3,063,681

TRANSFER OF HEAT FROM SUPERHEATED VAPOR IN A CONDENSING HEAT EXCHANGER Flled July 23, 1956 2 Sheets-Sheet 2 DRIVING 40 FORCE\ PUMP sucnon so E DJ &\ u. Q?

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' INVENTOR:

HOWARD Q. DUGUID HIS ATTORNEY United States Patent 3,063,681 TRANSFER OF HEAT FRGM SUPERHEATED VAPOR IN A CONDENSENG HEAT EX- CHANGER Howard Q. Duguid, Darien, Conn, assignor to Shell Oil Company, a corporation of Delaware Filed July 23, 1956, Ser. No. 599,435 2 Claims. (Cl. 257-40) The invention relates to an apparatus for transferring heat from a superheated vapor, such as superheated steam, to a heat-absorbing fluid in a heat exchanger which has a condensilng-wall one side of which is cooled by the said fluid while the vapor flows in contact with the other side of the wall and is condensed thereby. Heat exchangers in such condensing service are useful, for example, for imparting heat to a process stream in a chemical plant. Thus, the heat exchanger may serve as the reboiler at the bottom of a distillation column or be a part of a chemical reactor and disposed to maintain the reactants therein at a desired temperature.

Heat exchangers in condensing service generally show the highest overall heat transfer coefficient when condensing saturated vapor. Conventional design calls for introduction of the vapor to be condensed near the top and removal or" the resulting condensate near the bottom, the condensate being removed from the condensing-wall by gravity. With such designs difficulty is encountered when the vapor enters in the superheated condition as some portion of the condensing-wall surface must act as a vapor cooler, which function usually proceeds at lower heat transfer coefiicients. For small superheat the overall heat transfer coeflicient may not be reduced significantly, but for larger superheat it may be reduced so much as to require considerable additional, costly, condensing surface to attain a required heat flow. A further disadvantage of such an operation is that somewhat higher skin temperatures prevail in the condensing region of the wall; with some vapors, this accelerates corrosion.

When vapor is condensed on the shell side of a shelland-tube heat exchanger, the above difiiculty can be overcome by admitting the superheated vapor near the bot tom, just above the point of condensate removal, so that condensate, falling as droplets, desuperheats the entering vapor. However, when the vapor is to be condensed on the tube side, i.e., within the tubes, this solution is not readily applicable. If the heat exchanger contains vertical, single-pass tubes of sufficient cross sectional area to permit countercurrent two-phase flow, the vapor can be admitted at the bottoms of the tube for upward flow while the condensate flows downward. For small tubes, horizontal arrangements, and multi-pass arrangements, a separate desuperheating arrangement becomes necessary. While the invention is particularly useful in the instances noted in the preceding sentence, it can be applied also to the other arrangements mentioned in this paragraph.

It is, for various reasons, frequently not feasible to operate a condensing heat exchanger with saturated vapor; thus, the vapor may be superheated steam from a plant steam boiler which also drives a turbine or may be exhaust steam from a turbine which is still superheated. Desuperheaters are known in steam power generation systems and are costly and complex because they include condensate pumps which supply condensate to the pipes carrying the superheated steam, usually through spray nozzles. Because the quantity of condensate actually injected into the steam must be controlled, complex ratio controllers based upon the rate of steam flow and reset by the resultant temperature are essential components; also, liquid separators and high-temperature alarms are usually required. Such elaborate desuperheating installations are not usually economically justified for use with condensing heat exchangers.

It is, broadly, an object of the invention to provide a simple apparatus for transferring heat from a superheated vapor to a heat-absorbing fluid, employing a heat ex changer in condensing service, wherein superheat is removed from the superheated vapor by means of a coolant, such as condensate derived from the condenser. A specific object is to control the quantity of coolantsupplied to the superheated steam in accordance with the rate of steam flow, advantageously so that no coolant flows at minimum rates of steam flow.

In summary, according to the invention, the superheated vapor is passed through a vapor-actuated pumping device, such as an injector or difiiusor, and is, prior to flow into the heat exchanger, mixed with a liquid coolant, preferably condensate from the heat exchanger, which coolant is drawn into the superheated vapor stream by the said pumping device to efiect partial or complete removal of superheat. The coolant should, for the attainment of greatest heat economy, be substantially at the temperautre at whcih it is discharged from the heat exchanger. This liquid coolant is preferably accumulated in a surge tank or condensate-receiver at a level such that the coolant will not fiow into the steam line until operation of the pumping device. Since the pumping device is operated by the flow of steam, it is evident that the need for a control system which will sense the rate of steam flow and regulate the rate of coolant flow is obviated.

Although the invention is not strictly limited thereto, the type of vapor-actuated pumping devices which are of especial utility in the instant invention are those which, like the injector and diitfusor, have no moving parts for driving the pumped liquid and which commingle the pumped coolant with the vapor which is the driving medium; such devices are, for brevity, herein called static,

fluid-actuated, mixing pumps. It will be understood that.

such static pumps may be either controlled or uncontrolled, i.e., they may optionally be provided with adiustable means for controlling the flow of the fluids to maintain an efficient ratio. Thus, an uncontrolled injector may be merely a venturi constriction with a suction intake at the throat, or a convergent-divergent pressurizing nozzle extending through a suction intake chamber into a diverging tube which communicates with the chamber. A controlled injector would be fitted with a control plug entering into the throat of the constriction or nozzle and operated automatically in response to the flow rate so as to control the flow of both vapor and liquid while retaining an eflicient ratio.

In the arrangement according to the invention it will not be necessary to produce a high permanent pressure drop across the injector because the liquid coolant, viz., the condensate, will be lifted only through a small head, usually a few feet. The coolant will not begin to flow until the suction generated in the injector exceeds this head. Various arrangements for controlling this head in relation to the vapor pressure at the discharge of the injector are described below.

The invention will be described in greater detail with reference to the accompanying drawings forming a part of this specification and showing two preferred embodiments by way of example, wherein:

FIGURE 1 is a diagrammatic view of one embodiment, parts being shown in cross section;

FIGURE 2 is a draft indicating the pressure relationships of various flow rates; and

FIGURE 3 is a diagrammatic view of a second embodiment.

In FIGURE 1, 5 represents the shell of a heat exchanger having a tube bundle 6, shown a a bent tube or hair pin type. The tubes are mounted to a tube sheet 7 so that their opposed ends communicate respectively with with the chambers above and beneath a partition 8. A vapor, such as superheated steam form a source represented as a boiler 9, is admitted by a supply line 10 into the upper chamber, is condensed while passing through the tubes 6, and the resulting condensate flows into the lower chamber, from which it drains through a nozzle 11 into a condensate-receiver or seal pot 12. The heat exchanger may be connected to any suitable unit, such as a fractionating column 13, to which it is connected by a liquid-pipe 14 and a vapor-pipe 15. As is understood in the art, the heat exchanger in this service functions a a kettle reboiler, in that liquid from the bottom the column 13 enters the shell and functions as heat-absorbing fluid surrounding the tubes 6; the resulting vapors return to the column through the vapor pipe 15.

As was previously explained, the heat transfer coefiicient between the vapor, such as steam, and the inner wall of the tube 6 is low when the vapor enters the tubes in the superheated 'state. The desuperheating of the vapor generated in the plant 9 is, however, frequently not feasible because such vapor is also used in other units of a plant. Now, in accordance with this invention, the supply of superheated vapor through the heat exchanger without loss of heat is eifected by passing the steam through a vapor-actuated pumping device, e.g., a venturi or injector 16 having a pressure intake 17, a suction intake 18 at the throat thereof, and an outlet 19, and desuperheating the steam by commingliug therewith a portion of the condensate from the receiver 12, the said condensate being drawn in through a connecting pipe 20. This pipe is optionally provided with a flow-regulating device, such as a valve 21, and/or an orifice 22. Moreover, when the receiver 12 is of insufficient height it is desirable to install a liquid seal 23. Condensate in excess of that recycled through the line 20 is discharged through pipe 24, which may be provided with a valve 25 actuated by a liquid level cont-roller 26,

Operation is as follows: The flow of steam to the unit is controlled by valve 27 in accordance with the demand for seat in the fractionating column, by well-known control elements, not shown. At low heat demands, corresponding to low rates of superheated team flow through the pumping device 16, a low heat trans-fer coefiicient is acceptable; under these conditions the suction at the inlet 18 is insufficient to pump condensate through the pipe 20, and steam enters the tubes 6 in the superheated state. At higher rates of steam flow the suction increases until it exceeds the head H between the surface of the liquid in the receiver 12 and the axis of the venturi. At steam flow rates above that last mentioned, liquid condensate is sucked into the steam stream to remove superheat therefrom. Essentially saturated steam, or steam having only a slight degree of superheat, then enters the exchanger through the inlet pipe 10.

The pressure relations involved in the operation are indicated in FIGURE 2 wherein pressure heads are plotted as ordinates and flow rates are plotted in arbitrary units as abscissae. Curve A, marked Ap across system, shows the total pressure drop between the inlet 17 and the liquid level in the receiver 12 at varying flow rates, and is taken as one foot atunit flow rate. Curve B, marked pump suction, shows the diiference between the pressure in the inlet 17 and that at the suction inlet 18; it is plotted on an ordinates scale which is shifted downwards by a distance equal to the lift H. The pump suction is indicatcd as having a suction head of two feet at unit flow rate. It is evident that until a flow rate sufiicient to overcome both the lift H and the Ap across system is reached, no condensate is recycled; this condition prevails for all flow rates below about 2.8 on the graph, at which the curves intersect. For steam flows in excess of this liquid condensate will be aspirated into the venturi to afiect superheating.

It will be noted that the curve B rises more steeply with increasing steam flow than the curve A, so that the available driving force for the return of condensate increases. This tends to cause rates of condensate recycling which increase more rapidly than the steam rate, whereby the ratio of recycled condensate to superheated steam becomes progressively higher at increasing flow rates. Since it is usually undesirable to recycle condensate in amounts beyond that required to produce saturated steam, a throttling device such as adjustable valve 21 and/ or an orifice disc 22 is provided in the pipe 20, Because the pressure drop across such a flow-restricting device increases for increasing flow rates, it is effective in preventing the recycling of excessive amounts of condensate.

It will be noted from the foregoing that, for a given steam supply pressure at the inlet 17, the pressure in the receiver 12 and, hence, that at which the condensate is supplied to the suction intake 18, is less than the pressure of the steam at the inlet 17 by a difference which, at zero steam flow, is equal to the head H. This difference increases by an additional amount due to pressure drop, as represented by the curve A (ignoring pressure drop at 21 and 22 at increasing steam flow rates. Since the pressure at the ump outlet 19 is only slightly less than that at the inlet 17 this relation is also substantially valid between the intake 18 and the outlet 19.

It is evident that numerous changes in the arrangement as well as substitutions in the components thereof may be made within the spirit and scope of the invention. Thus, the vapor-actuated pump may take the form of a diffuser such as that disclosed in British Patent No. 648,980.

FIGURE 3 illustrates variants both in the heat exchanger tubes and in the arrangement for returning condensate to the steam. In this view, the heat exchanger includes several series of bayonet heaters 30, 31 and 32 (only one such series being shown) supplied with steam through a header 33. These heaters may, for example, extend into a vessel 29 containing the heat-absorbing fluid. Each heater contains an insert pipe 34 by which steam is admitted and supplied to the inner end of the heater housing, which is secured to a terminal section 35 having a discharge nozzle 36. The outlet nozzle 36 of the last heater in each series is followed by an air vent 37 located just above the condensate interface to bleed off inert gas which would otherwise accumulate above the seal leg. The interface may be at atmospheric or higher pressure. Condensates from the several series flow through pipes 38 to a condensate header 39 and into a condensate-receiver 40 via a pipe 41. The receiver functions as a separator tank and is connected by pipes 42 and 43 to an auxiliary level control tank 44 from which condensate is discharged through a pipe 45 at a rate controlled by a float-controlled valve 46, Superheated vapor, such as steam, from a generator 47 is supplied to the inlet 48 of a venturi or injector 49 and is discharged via the outlet 50 thereof to the upper part of the condensate receiver, which is further connected to the steam header 33 by a steam pipe 51. The suction inlet 52 of the injector is connected by a dip-pipe 53 to the lower part of the receiver 46 and contains a flow-restrictive device, such as a valve 54. It will be noted that the condensate header 39 is below the level L of the liquid in the receiver 40. The pressure drop Ap through the tubes, shown in the drawing, corresponds to the height to which condensate accumulates in the pipe 38 above the level L. The height H of the suction inlet 52 above the level L can in this case be made small, such as zero; in the preferred -arrangement illustrated it may be from one to several inches.

In operation superheated steam flowing through the injector 49 is comrningled with condensate as soon as the rate of steam flow is suificient to generate the suction head equal to the head H. It will be noted that in this embodiment liquid from the receiver 40 is supplied to the pump intake 52 at a pressure which is less than that of the pump discharge 50 by a substantially constant difference (ignoring any pressure drop through the device 54) because the pump discharges into the upper part of the receiver 40. The flow restrictive device 54 plays the role previously noted for the valve 21.

It will be noted that, in both embodiments, the liquid returned to the superheated vapor is condensate which is substantially at the temperature at which it was discharged from the heat exchanger. Consequently, there is no overall loss of heat and the invention permits the attainment of an improved heat transfer coefficient in the exchangers with but a slight loss of pressure through the pump.

I claim as my invention:

1. In combination with a heat exchanger having a condensing-wall one side of which is cooled by a fluid to be heated: a condensate-receiver connected to said exchanger to receive condensate from the other side of said condensing-wall; means for maintaining said receiver partly filled with liquid; a vapor-actuated pumping device having a pressure inlet for driving vapor, a suction inlet connected to receive liquid from said receiver, and a discharge connected to transfer the combined feed to the pumping device into the upper part of the said receiver; a source of superheated vapor connected to said pressure inlet and flow means for passing vapor from the upper part of said receiver through the heat exchanger in contact with the said other side of the said wall.

2. In combination with a heat exchanger having a condensing-Wall one side of which is cooled by fluid to be heated: a vapor-actuated pump having a pressure inlet for driving-vapor, a suction inlet, and an outlet, :1 source of superheated vapor connected to said pressure inlet; flow means fo passing effluent vapor from said outlet through the heat exchanger in contact with the other side of said condensing-wall; a condensate receiver having means for discharging liquid therefrom while maintaining a body of liquid therein; said receiver being in condensate-receiving flow communication with the said flow means so as to receive condensate substantially at the temperature of discharge thereof from the heat exchanger; conduit means interconnecting said condensate-receiver at a level therein below the level of said body of liquid directly with said suction inlet, whereby condensate from the receiver flows at essentially constant temperature; said condensatereceiver being in direct flow-communication above the said liquid level with the outlet of the pump; and flow communication means for commingling the condensate which is drawn into said suction inlet with said superheated Vapor before flow thereof in contact With the condensingwalls to remove superheat therefrom.

References Cited in the file of this patent UNITED STATES PATENTS 1,886,590 Rosenblad Nov, 8, 1932 2,069,653 Hosel Feb. 2, 1937 2,537,259 Cleaver et al. Jan. 9, 1951 2,576,843 Lockman Nov. 27, 1951 2,619,453 Anderson Nov. 25, 1952 2,685,280 Blaskowski Aug. 3, 1954 2,822,784 Heller Feb. 11, 1958 

